Today's He--Ne laser gyros dominate the inertial navigation market world-wide. Such laser gyros, commonly known as ring laser gyros or RLG's, are approaching a ceiling in technical development and there is relatively little room left for improvement. It is therefore a primary motivation of the instant invention to provide alternate conceptions of ring laser gyros, rather than to attempt to push further the limits of a technology which is nearing perfection.
Important goals to be achieved for the next generation of rotational sensors include:
Smaller size; PA1 All solid state design; PA1 Reduced noise (hence, higher accuracy); and PA1 Lower cost.
The first two goals suggest the use of integrated opto-electronics. Unfortunately, the use of solid state lasers implies generally large scattering, hence a large dead band, and large homogeneous broadening, which tends to make ring laser operation unidirectional. The problems associated with a large dead band are discussed further immediately below as they relate to the "lock-in" phenomenon.
Ring laser gyros typically include a beam path, usually some form of cavity, along which two oppositely traveling laser beams propagate. A common problem in all ring laser gyros is known as "lock-in" which results in an undesirable dead band characteristic. As a ring laser gyro is rotated, the cavity round trip time or path length becomes different for the two oppositely traveling beams. This implies that the two oppositely traveling beams have to assume different frequencies. If portions of each of the beams are allowed to exit the cavity and are mixed, a beat frequency can be detected that is proportional to the applied rotation rate. This is the ideal laser gyro. Unfortunately, due to backscattering of one of the beams into the other there is a coupling of the two oppositely traveling beams. At low rotation rates this coupling causes both of the oppositely traveling beams to assume the same frequency and the beat frequency disappears. This frequency synchronization of the oppositely traveling beams is termed "lock-in".
U.S. Pat. No. 4,525,843 to Diels provides a phase conjugated coupling between the counter propagating beams of a ring laser which reduces the lock-in threshold and makes an homogeneously broadened solid state laser a suitable for use as a laser gyro. As a result, it is possible to make a compact active laser gyro with solid state and optical integrated circuit lasers.
As can be seen from the above discussion, the coupling between counterpropagating beams in the gyro is responsible for the lock-in phenomenon. It is a well established fact that, in short pulse lasers (i.e. a laser having a pulse width substantially shorter than the path length), the coupling between counterpropagating beams is restricted to the region of overlap of the counterpropagating beams or pulses. The overlap region can be very small if the circulating energy is concentrated in ultra-short pulses. This property was of little practical importance when investigated in the late 1960's because the shortest laser pulses available at the time were of length comparable to that of the laser cavity. This approach has to be reconsidered now that ultra-short pulses can be generated with a spatial extension in the micron range.
Ideally, the pulse overlap is to occur in a region of minimal scattering. A primary object of the instant invention is thus to prevent the pulses from overlapping in the laser gain medium which is a region of maximum scattering. One possibility used in the prior art employed a type of coupling to increase the overlap probability of two continuous wave beams without a tendency to lock their phases. Degenerate four wave mixing provides such a coupling. Saturable absorption is another means of forcing the two pulses to meet at a specified location. Therefor the invention is motivated by the need to precisely time the propagation of a set of pumped pulses to avoid lock-in and backscatter.